A F/4 B F/8 C 2F D 4F E 8F. Answer: Because F M A. /r 2 or eight times what it was 8F. Answer:

Transcription

1 Test 7 Section A 2 Core short answer questions: 50 marks Section B 2 Detailed studies short answer questions: 120 marks Suggested time: minutes Section A: Core short answer questions Specific instructions to students QUESTION 1 What is the mass of a satellite if it orbits the Earth at an altitude of 550 km and the Earth exerts a force of N on it? The gravitational force on two objects (A and B) in space is F newtons when their centres are separated by a distance r kilometres. QUESTION 2 What is the size of the new gravitational force acting on the objects if the distance between their centres is doubled to 2r kilometres? Explain your answer. A F/4 B F/8 C 2F D 4F E 8F Answer all questions in the spaces provided. For all questions which require a numerical answer you must show all working. You should take the value of g to be 10 ms 22. F , r , M Earth , M satellite 5? F 5 GM M Earth satellite r 2 M satellite 5 (Fr 2 ) 4 (GM Earth ) M satellite 5 ( ( ) 2 ) 4 ( ) kg kg A kg Because F 1/r 2, F becomes 1/(2r ) 2 or ¼, what it was F/4. QUESTION 3 What is the size of the new gravitational force acting on the objects if the mass of object A is doubled and the distance between their centres is halved? A F/4 B F/8 C 2F D 4F E 8F E Because F M A /r 2, F becomes 2M A /(r/2) 2 5 8M A /r 2 or eight times what it was 8F. QUESTION 4 A Physics teacher tells his class that an object travelling in circular motion at constant speed does not have constant momentum. Is this statement true or false? Explain your answer. This statement is true. Momentum is a vector quantity. In uniform circular motion, the object s velocity is constantly changing direction, therefore its momentum is also constantly changing direction. Momentum therefore cannot be constant. A cricketer catches a 150 g cricket ball and brings it to rest from a speed of 16 ms 21 over a distance of 12 cm with a nearly constant stopping force. QUESTION 5 Determine the time it took the cricketer to stop the cricket ball. u 5 16, v 5 0, s , t 5? s 5 ½ (u 1 v)t ½ t t s s Test 7 35

2 QUESTION 6 Calculate the magnitude of the impulse of the cricket ball during the catch. I 5 p 5 m v kgms kgms 21 QUESTION 7 What average force did the cricketer exert on the cricket ball during the catch? I 5 F av 3 t F av 5 I 4 t N An 82 kg stunt pilot flies his plane in a vertical circle of radius 120 m at a constant speed of 43 ms 1. QUESTION 10 What is the size of the centripetal acceleration of the pilot and his plane during this motion? a c 5 v 2 /r or 15 ms 2 15 ms 22 At the top of the loop, the pilot and his plane are upside down. QUESTION 11 What is the size of the force acting between the pilot and his seat at the top of the loop? 160 N QUESTION 8 What amount of work the cricketer does on the cricket ball during the catch? F c F seat W W 5 F av 3 s QUESTION 9 The cricketer finds that his hands hurt after catching the cricket ball. What advice would you give him to reduce the size of the stopping force the next time he takes a similar catch? J or 19 J OR W 5 KE 5 ½ mv 2 19 J 5 ½ J or 19 J Increase the stopping distance from 12 cm by giving more with his hands. For example, making the ball stop over a distance of 36 cm will reduce the force by a factor of 3. F c 5 F seat 1 W F seat 5 F c 2 mg 440 N QUESTION 12 How long does it take for the plane to complete one full loop? v 5 2pr /T T 5 2pr/v T 5 2p or 18 s 18 s 5 ma c 2 mg or 440 N QUESTION 13 At the top of the loop, what would be the direction of the plane s instantaneous velocity? A B C D zero C During the circular motion, the direction of the instantaneous velocity is always at a tangent to the motion. 36 A1 Physics Exam 1 VCE Unit 3

3 QUESTION 14 A person of mass 65 kg is in a lift travelling upwards and accelerating upwards at a rate of 0.5 ms 2. What is his apparent weight during this motion? N 5 ma 1 W N or 683 N An electric circuit consists of three resistors connected in parallel and a 9 V supply voltage, as shown below. 9 V QUESTION 18 What is the size of the current at point A in this circuit? I 5 V 4 R A 1.8 A An LED circuit has the following circuit diagram. 1.5 V A R QUESTION 15 Calculate the effective resistance of this circuit. R effective 5 ( ) QUESTION 19 If the current through the LED is 40 ma and the potential difference across it is 1.4 V, calculate the value of the resistor R. R 5 V 4 I 5 ( ) 4 ( ) A student designs a circuit that will automatically turn on a set of low-voltage outdoor lights after sunset. She uses an LDR whose resistance increases as the light intensity falling on it decreases. The circuit appears below. QUESTION 16 Calculate the size of the current flowing through the 15 resistor. +6 V R 1 I 5 V 4 R A LDR to light circuit 0.6 A QUESTION 17 Calculate the power output from the 10 resistor. P 5 V 2 4 R W 8.1 W 0 QUESTION 20 Will the circuit work correctly and turn the lights on as it starts to get dark? Explain your answer. Yes, the circuit will operate correctly. As the light intensity drops, the resistance of an LDR increases and therefore the voltage across it and to the light circuit also increases, turning the lights on. Test 7 37

4 QUESTION 21 If R 1 has a value of 1000 and the LDR has a value of 2000 at a particular light intensity, what output voltage is provided to the light circuit? V out 5 R 2 4 (R 1 1 R 2 ) 3 V in V out ( ) V 4 V QUESTION 22 With the same light conditions as described in question 21, and the light circuit turned off, what current normally flows through the LDR? Give your answer in ma. I 5 V/R A 5 2 ma 2 ma Section B: Detailed studies short answer questions Specific instructions to students Answer the following questions for ONE detailed study only. Detailed study 1: Einstein s special relativity QUESTION 2 How far does the traveller onboard the spacecraft see the distance from the Earth to Alpha Centrauri to be? Explain your answer. L 5 L o / y QUESTION 3 How much time does the Earth-bound observer notice passing on the spacecraft s clock during its journey from the Earth to Alpha Centauri? Explain your answer light years Explanation: In the frame of reference of the traveller, the distance from the Earth to Alpha Centauri is length contracted. Hence, he sees the distance as shorter than it really is. 0.4 light years 0.4 light years The Earth-bound observer sees the spacecraft s clock run slow by a factor of y = 10. Therefore, he sees it only takes 0.4 years to travel from the Earth to Alpha Centauri. The following diagrams represent the apparatus used by Michelson and Morley in their famous 1887 experiment. QUESTION 4 Trace with a solid line, the path of light from the source S to M 1 and then to the observer. Relevant data: Speed of light c ms 21 Rest-mass of an electron kg The star Alpha Centauri is approximately 4 light years in distance from the Earth. A traveller on board a spacecraft travels from the Earth to Alpha Centauri at a constant speed of 0.995c. QUESTION 1 Calculate the value of the Lorentz factor for the spacecraft described above. g v 2 10 c ( ) 1/ S M 1 observer M 2 aether wind 38 A1 Physics Exam 1 VCE Unit 3

5 QUESTION 5 Trace with a dotted line, the path of light from the source S to M 2 and then to the observer. QUESTION 9 How did Lorentz account for Michelson and Morley s result? S M 1 aether wind A The speed of light is absolute and independent of the frame of reference of the observer. B The aether was carried along with the Earth. C The apparatus was not sensitive enough. D The arm of the experiment that was perpendicular to the aether wind had been length contracted. observer M 2 QUESTION 6 4 marks The previous diagrams have a label of aether wind. Explain what the aether wind was meant to be and why it was considered to be important. The aether wind was thought to be caused by the motion of the Earth through aether. Just as the speed of sound was affected by wind, the speed of light was thought to be affected by the aether wind. Statement D is correct. Lorentz came up with the Lorentz factor to explain the null result of the Michelson Morley experiment. A train carriage moving at close to the speed of light has a special device located at its centre. This device can send out a beam of light in the forward direction at the same time as it sends out a beam of light in the rearward direction. direction of travel QUESTION 7 The previous diagram shows Michelson and Morley s experimental set-up. How did they perform the actual experiment? Michelson and Morley observed the interference effect of the two light beams, then rotated the whole apparatus through 90 and looked again at the interference effect. (No difference in the pattern was observed.) QUESTION 8 The main reason why the Michelson2Morley experiment failed in its aim was because: A The speed of light is absolute and independent of the frame of reference of the observer. B The aether was carried along with the Earth. C The apparatus was not sensitive enough. D The arm of the experiment that was perpendicular to the aether wind had been length contracted. Statement A is correct. This was Einstein s explanation of the null result of the Michelson Morley experiment. QUESTION 10 Which of the following would be correct for an observer inside the train carriage? A The forward travelling beam will strike the forward end of the carriage before the rearward beam strikes the rearward end of the carriage. B The forward travelling beam will strike the forward end of the carriage after the rearward beam strikes the rearward end of the carriage. C The forward travelling beam will strike the forward end of the carriage at the same time as the rearward beam strikes the rearward end of the carriage. D It is impossible to tell which beam of light will strike the end of the carriage first. Answer C is correct. To an observer inside the carriage, the two events are simultaneous. They are only not simultaneous to a stationary observer outside the carriage when the carriage travels past at near the speed of light. Test 7 39

6 QUESTION 11 Which of the following would be correct for a stationary observer located outside the train carriage, standing opposite the centre of the carriage when the beams are turned on? A The forward travelling beam will strike the forward end of the carriage before the rearward beam strikes the rearward end of the carriage. B The forward travelling beam will strike the forward end of the carriage after the rearward beam strikes the rearward end of the carriage. C The forward travelling beam will strike the forward end of the carriage at the same time as the rearward beam strikes the rearward end of the carriage. D It is impossible to tell which beam of light will strike the end of the carriage first. Answer B is correct. In the time between the beams being turned on and their arrival at the observer s position, the carriage has travelled forwards. This has brought the rear end of the carriage closer to the observer. Hence, this beam has a shorter distance to travel and the observer sees it strike the rearward end before the forward beam strikes the forward end of the carriage. QUESTION 12 The situation explained in question 11 is an example of: A the relativity of simultaneity B length dilation C time dilation D mass dilation Answer A is correct. This is an example of the relativity of simultaneity. An alien spacecraft is 15 m long when at rest. It travels through space at 0.4 c. QUESTION 14 How much long will the spacecraft appear to be to a stationary observer located outside the spacecraft? g v 2 c ( ) L 5 L o /y m or 13.8 m 13.8 m QUESTION 15 How long will it appear to take the alien to travel a distance of 500 km in the frame of reference of the alien? The alien sees the distance 500 km contracted to km. Therefore, time taken 5 distance 4 speed ( ) s s QUESTION 16 How long will it appear to take the alien to travel a distance of 500 km from the frame of reference of the stationary observer? For the stationary observer, there is no length contraction. So the time taken 5 distance 4 speed ( ) s s Detailed study 2: Materials and their use in structures For the next five questions, use arrows to represent the forces acting on each object. QUESTION 13 How long will the spacecraft appear to be to its alien pilot? QUESTION 1 Shear forces: 15 m The alien pilot sees the proper length of the spacecraft. 40 A1 Physics Exam 1 VCE Unit 3

7 QUESTION 2 Tension forces applied to both ends: QUESTION 3 Bending forces: Answer A is correct. Because the compressive modulus 5 (F/A) 4 ( L/L), if F increases the stress will increase, but the length will increase too, keeping the compressive modulus constant. QUESTION 4 Compression forces applied to both ends: QUESTION 8 If the compressive strength of the metal in the rod is Pa, at what compressive force would the rod fail? F 5 s 3 A or 9800 N 9800 N QUESTION 5 The weight force acting from the centre of mass: A 6.2 m length and 0.70 mm diameter fishing line has a Young s modulus of Pa. QUESTION 9 4 marks By how much will the fishing line stretch when it is subjected to a tensile force of 46 N? A metal rod of length 94 cm and diameter 3.6 mm is subjected to a compressive force of 8300 N. Under this load, the rod compresses to a length of 93.4 cm. QUESTION 6 4 marks Calculate the value of the compressive modulus for the rod. Compressive modulus 5 s/ 5 (F/A) 4 ( L/L) A 5 pd 2 /4 5 p 3 ( ) m 2 Compressive modulus 5 ( ( )) 4 (( ) 4 ( )) Pa Pa QUESTION 7 If the compressive force is increased to N and all else remain the same, which of the following would be true? A Compressive modulus remains constant and the stress increases. B Compressive modulus and the stress both remain constant. C Compressive modulus and the stress both decrease. D Compressive modulus remains constant and the stress decreases. Y 5 s/ QUESTION 10 If the fishing line has a breaking strength of Pa, will the 46 N tensile force be large enough to break the fishing line? Justify your answer. 5 (F/A) 4 (DL/L) A 5 p 3 ( ) m 2 DL 5 (F/A) 4 (Y/L) 5 (46 4 ( )) 4 (( ) 4 6.2) m 0.20 m s 5 F/A ( ) Pa (less breaking strength) Therefore, no. No Test 7 41

8 A stress2strain graph for the fishing line appears below. (x 10 8 Pa) QUESTION 11 Indicate on the graph the yield point. What is the approximate value of the yield stress? elastic Pa fracture point plastic region yield point When designing structures, engineers often incorporate a safety factor or factor of safety into their calculations. QUESTION 17 What is meant by the term safety factor? A safety factor is a factor built into the design that effectively lowers the maximum allowed working stress by the amount of the factor. QUESTION 18 If a particular material has a breaking strength of Pa, and a safety factor of 10 is used in the design, what would be the maximum working stress allowed for this material? Maximum allowed stress 5 breaking stress 4 safety factor Pa QUESTION 12 Indicate the elastic region on the graph Pa QUESTION 13 Indicate the plastic region on the graph. QUESTION 14 Indicate the fracture point on the graph. A wooden sign of mass m kg is suspended from the roof by two metal wires as shown in the diagram below. 60 roof QUESTION 15 Calculate the volume of the 6.2 m length of fishing line. V 5 A 3 length QUESTION 16 4 marks Calculate the amount of strain energy stored in the fishing line just prior to fracture m m 3 Strain energy 5 area under graph 3 volume Area under graph 5 ½ ½(4 1 10) Jm 23 Strain energy J accept from J SIGN QUESTION 19 What will be the size of the tension force in each wire? A mg/2 B mg/(2sin60 ) C mg/(2cos60 ) D mg/(2tan60 ) Answer B is correct. The diagram shows the forces acting on the sign: mg down and two tension forces (T ). sin 60 5 (mg/2) 4 T T 5 mg/2sin60 mg 2 T 60 T 42 A1 Physics Exam 1 VCE Unit 3

9 QUESTION 20 What will be the size of the total force applied to the roof by the two wires? A 2mg B mg C mg/2 D mg/4 Detailed study 3: Further electronics QUESTION 1 On the axes below, sketch the appearance of two cycles of an AC voltage that has a peak-to-peak voltage of 12 V and a period of 4 ms. V (V) Answer B is correct. The two tension forces add up to mg downwards. T T mg QUESTION 3 Would a multimeter or a cathode-ray oscilloscope be best to measure this current? Explain your answer. A multimeter would be best. A CRO doesn t measure current. QUESTION 4 In order to have a time constant of 0.01 s, what size capacitor would be used with the 5 k resistor? Give your answer in F. 5 RC C 5 /R ( ) F 2 mf For the next two questions, choose your answer from among the following alternatives: A B time (ms) -6 C D A fully charged capacitor has a voltage of 10 V across its plates. It is discharged through a 5 k resistor. QUESTION 2 What current initially flows through the resistor? I 5 V 4 R ( ) A 5 2 ma 2 ma QUESTION 5 Which of the above graphs best shows the voltage across the capacitor as a function of time as the capacitor discharges through the 5 k resistor? Answer C is correct. This graph shows the voltage starts high and drops by 63% in one time constant. Test 7 43

10 QUESTION 6 Which of the above graphs best shows the current flow through the 5 k resistor as a function of time as the capacitor discharges? Answer C is correct. This graph shows the current starts high and drops by 63% in one time constant. The diagram below represents the current flow from a rectifier circuit being passed through a load resistor, R L. The signal has a maximum peak voltage of 9.20 V at a frequency of 50.0 Hz. A capacitor can be used in order to smooth the signal from the rectifier circuit before it passes through the resistor. from the rectifier R L For the next two questions, choose your answer from among the following alternatives: A C B D QUESTION 9 Draw on the diagram above to show where the capacitor would be connected in order to achieve the best smoothing of the signal. QUESTION 10 If the load resistor, R L, has a value of 110, what peak current flows through the 110 load resistor? I P 5 V P 4 R A A A QUESTION 7 Which of the above diagrams shows the best diode arrangement in order to achieve half-wave rectification? Answer A is correct. Half-wave rectification is achieved by a single diode. QUESTION 11 What peak power is dissipated in the 110 load resistor? P P 5 V P 3 I P W 0.77 W QUESTION 8 Which of the above diagrams shows the best diode arrangement in order to achieve full-wave rectification? Answer C is correct. Full-wave rectification is achieved by a diode bridge. A student drew the following block diagram of the components needed to make an AC to DC voltageregulated power supply system. AC power supply transformer A smoothing load QUESTION 12 What is the role of the transformer? The transformer converts the AC power supply voltage to a lower value. 44 A1 Physics Exam 1 VCE Unit 3

11 QUESTION 13 If the power supply is 240 V AC and the transformer has 1000 primary turns and 50 secondary turns, what is the output voltage of the transformer? V 2 /V 1 5 N 2 /N 1 V V QUESTION 14 Identify component A and outline its role. 512 V Component A is a rectifier. Its role is to convert the AC signal into either full-wave or half-wave rectified DC. QUESTION 18 In a correctly regulated DC power supply, the effect of an increase in the power supply voltage would cause (one or more answers): A an increase in the voltage across the load. B an increase in the current flowing out of the AC power supply. C no change in the voltage across the load. D an increase in the current through the load. Answers B and C are correct. The current flowing out of the AC power supply would increase but the voltage and current through the load remain constant. QUESTION 15 Indicate which major component is missing from the block diagram and where on the diagram it should be placed. The missing component is a voltage regulator. It should be inserted between smoothing and the load. QUESTION 16 Outline the role of the component identified in question 15. The role of the voltage regulator is to keep the voltage constant across the load resistor despite fluctuations in the supply voltage. QUESTION 17 The minor component of a heat sink could also be incorporated into the block diagram; however, it is not usually located on its own. Identify which component is the most likely to require a heat sink and explain why it needs it. The heat sink is usually located with the voltage regulator. Its job is to dissipate the heat generated because of the large amount of current flowing in the regulator. Test 7 45